Cation-Dominated Second-Order Nonlinear Optical Response Enabled by π-Conjugated Guanidinium Derivatives

Area of Science:

• Materials Science
• Crystallography
• Optics

Background:

• Rational design of ultraviolet (UV) nonlinear optical (NLO) crystals requires understanding structure-property relationships and identifying key functional units.
• Guanidinium-based compounds are explored for their potential in NLO applications.

Purpose of the Study:

• To report a novel hybrid UV NLO crystal, GALSO3CH3, with a cation-dominated NLO response.
• To investigate the activation of NLO activity in the guanidino-acetic lactam (GAL+) cation via molecular orbital engineering.
• To establish guanidinium-derived cations as effective NLO-active building units.

Main Methods:

• Synthesis of the hybrid crystal GALSO3CH3.
• Intramolecular cyclization of guanidino-acetic acid to form the GAL+ cation.
• First-principles calculations to analyze electronic structure and NLO contributions.
• Characterization of optical properties including UV cutoff and birefringence.

Main Results:

• The GAL+ cation exhibits enhanced π-electron delocalization, molecular polarity, and polarizability anisotropy.
• GALSO3CH3 shows a short UV cutoff edge (~214 nm) and moderate birefringence (~0.06).
• The crystal demonstrates a large, phase-matchable second-harmonic generation (SHG) response, approximately four times that of KH2PO4.
• First-principles calculations confirm over 90% of the SHG response originates from the π-conjugated GAL+ cations.

Conclusions:

• Guanidinium-derived cations, specifically the GAL+ cation, are identified as potent NLO-active building units.
• A π-conjugation-enhanced strategy is effective for designing advanced UV NLO materials.
• The developed GALSO3CH3 crystal represents a significant advancement in UV NLO material design.